1. Field of the Invention
This invention relates generally to phased array antenna systems and more particularly to an improved beam forming matrix for forming a plurality of orthogonal beams.
2. Description of the Prior Art
Electrically scanned antennas are generally well known and comprise an antenna system including a plurality of radiating elements which are fixed in space and wherein one or more RF beams are simultaneously generated and moved by introducing a phase delay into the radiated wave front. Such an antenna, moreover, is called a phased array. One illustrative example of such a system is shown and disclosed in U.S. Pat. No. 4,028,710, entitled, "Apparatus For Steering A Rectangular Array . . . " which issued to G. E. Evans, the present inventor, on June 7, 1977.
The Butler matrix, moreover, since its inception has found wide applicability in the formation of such beams. A Butler matrix is well documented in the prior art and typically comprises a network of 3-db directional couplers and fixed phase shifters where the directional couplers are comprised of four port power dividers having the property of providing two outputs differing in phase by 90.degree., or conversely, of coupling all power to one of two isolated ports when power is applied equally to two other ports with a 90.degree. phase differential. Illustrative examples of this type of beam forming matrix, moreover, are shown and disclosed in U.S. Pat. No. 3,255,450, entitled, "Multiple Beam Antenna System Employing Multiple Directional Couplers In The Leadin", which issued to J. L. Butler on June 7, 1966, and U.S. Pat. No. 3,295,134, entitled, "Antenna System For Radiating Directional Patterns", which issued to W. R. Lowe on Dec. 27, 1966.
A Butler matrix, however, has an inherent limitation in that it comprises a binary network in that it can only be used for a binary number (2.sup.n) of antenna elements. All of the 2.sup.n outputs of the matrix are fed equally from the same number of 2.sup.n inputs with a linear phase front. Each phase front has a different slope across the outputs which change in steps of 2.pi./2.sup.n radians per element.
Where there is a requirement for other than a binary number of outputs, a suitable beam forming matrix can be developed but prior art design techniques require the utilization of a network which becomes relatively complex and physically awkward to implement in comparison to a binary matrix system. A typical example of non-binary matrix is shown and disclosed in U.S. Pat. No. 4,231,040, entitled, "Simultaneous Multiple Beam Antenna Array Matrix And Method Thereof", which issued to S. H. Walker on Oct. 28, 1980. The problem of designing a simple and efficient matrix becomes particularly difficult where less than the number of available beams are used and the desired beams result from a selected number of beams which are necessarily generated by a non-binary matrix. Such a situation exists, for example, where only a small sector of a total elevation region is utilized.
Accordingly, it is an object of this invention to provide an improved network for forming multiple beams in an antenna array.
It is another object of the invention to provide a simplified network for forming a set of orthogonal beams from a phased array.
A further object is to provide an antenna system whereby a binary beam forming matrix is expanded in such a manner that it is capable of being used in conjunction with a non-binary number of antenna elements.
And still a further object of the invention is to provide a simplified network whereby a binary matrix is transformed into a non-binary matrix for forming a plurality of component beams which are utilized to construct relatively larger composite beams having reduced side lobes.